1. Field of the Invention
The present invention relates to the field of photography and, more specifically, to improved photographic film packs for use in self-developing cameras and other related photographic apparatus.
2. Description of the Prior Art
The present invention is directed to certain improvements in photographic film assemblages or packs configured for use with compact, highly-automated self-developing cameras. These film packs include a film container or cassette having an exposure aperture in a forward wall or face and a film withdrawal slot at one end of the container adjacent the forward wall; at least one but preferably a plurality of "integral type" self-developing film units arranged in stacked relation behind the forward wall and biasing means, such as a spring platen, for urging the stack of film units toward the forward wall to locate the top film unit against the underside of the forward wall in a forwardmost exposure position. Subsequent to exposure, the forwardmost film unit is advanced through the withdrawal slot and into the bite of a pair of camera mounted pressure-applying rollers which apply a compressive pressure to the film unit as it is advanced therebetween to discharge a supply of fluid processing composition, from a pod at the leading end of the film unit, and distribute the fluid imagewise between a predetermined pair of layers in the film unit to cover the film unit's image forming area and initiate a well-known development and diffusion transfer process.
The "integral type" film unit is configured as a multilayer laminate and includes a transparent window through which the photosensitive material is exposed. After processing, the same window is used to view a positive image print. By the use of this construction, the need to peel apart separate negative and positive sheet elements after fluid distribution is eliminated.
Because the laminate sheet elements or layers are tightly bound together at their lateral edges, the predetermined pair of layers tend to separate more in the center of the film unit than out at the lateral margin in response to the fluid being distributed therebetween. In other words, there is more resistance to fluid flow at the edges of the film unit than in its center section. Upon initial discharge of the fluid from the pod, it assumes a rearwardly extending tongue shape rather than proceeding toward the trailing end of the film unit along a uniform wave front. Without additional spread control, an overabundance of fluid in the center portion of the film unit may lead to insufficient fluid to completely cover the trailing end corners of the film unit's image forming area. It is well known in the art of instant photography to apply a second compressive pressure to the film unit, behind the pressure-applying rollers, to a selected portion or center section of the film unit for the purpose of modifying the wave front so that it progresses in a more uniform manner toward the trailing end. This pressure selectively limits the gap between the predetermined layers at their center thereby retarding the center of the tongue shaped wave front and allowing the fluid to be diverted outwardly to the lateral margin areas where there is less resistance to fluid flow because of the absence of the second compressive force in those areas.
In the earlier years of self-developing photography, pressure was applied with a pressure plate located between the rollers and the film exit slot of the film container. In more recent times, the spread control structure for applying the second compressive force to the center of the film unit has been built directly into the film container and generally includes pressure applying structure located on the underside of the forward wall near the film withdrawal slot.
One example of such a contemporary self-developing film pack is the commercially available SX-70 Land Film, manufactured by Polaroid Corporation, Cambridge, Mass.
For an example of the spread control structure used therein, reference may be had to commonly assigned U.S. Pat. Nos. 3,779,770 and 3,948,662.
Briefly the underside of the forward wall near the film withdrawal slot has integrally molded triangular sections which thin down the forward wall near the lateral edges of the withdrawal slot. This structure is best shown in FIGS. 9, 9a and 9b of the two previously noted patents.
When the forwardmost film unit in the stack is urged against the underside of the forward wall by the spring platen, the relatively thick center section of the forward wall near the withdrawal slot presses on the center of the film unit advancing therethrough and into the bite of the pressure rollers while the thinner lateral portions of the forward wall defined by the triangular sections is subjected to a relatively small compressive force. In this manner, the second compressive force is applied selectively to the center and serves to modify the shape of the fluid wave front as previously described.
Another characteristic feature of the commercially available SX-70 Land Film pack is that it includes a molded plastic end cap over the film withdrawal slot which is moved from a slot blocking position to an unblocking position by structure in the self-developing camera in response to inserting the film pack into the camera's receiving chamber. Such an end cap is described and claimed in commonly assigned U.S. Pat. No. Re 29,756, reissued on Sept. 5, 1978. While the end cap does not play any part in controlling the distribution of the processing fluid, the reference is provided to more accurately describe the prior art film pack. Also, it will be found hereinafter that one of the improvements of this invention is to provide a film pack that does not require such an end cap.
The SX-70 Land Film pack is intended for use in a variety of self-developing cameras manufactured by the Polaroid Corporation, Cambridge, Mass. These include the SX-70, Pronto!, and OneStep Land cameras. For representative examples of these cameras, reference may be had to commonly assigned U.S. Pat. Nos. 3,714,879; 3,750,551; 3,979,762; 4,052,728; and 4,134,655.
Each of these cameras has a front pack loading door, mounting a pair of pressure-applying rollers, that is movable from an operative closed position to an open position that provides access to the open end of a film pack receiving chamber. Structure is provided in the receiving chamber for accurately locating the pack or film container at an operative position within the camera. Longitudinally, pack placement is defined by a fixed rear stop in the camera and a leading end stop mounted on the movable loading door section.
Because the distance between the stops is fixed, there is a limit as to just how much an improved film pack may be modified. In other words, dimensional considerations of the camera must be considered so that any modification of the pack or film container will be compatible with existing cameras in the hands of consumers.
While the spread control system of the commercially available pack works very well, it has been found that on a statistical basis there is some room for improvement. More specifically, while there are very few instances in which the trailing end corners of the image area are not covered, analysis of the distribution of the fluid under the lateral margin binder and in the trap area at the end of the film unit indicates that the efficiency of fluid distribution could be improved to the point where it may be possible to provide a smaller volume of fluid with each film unit thereby leading to a significant cost reduction. Also, statistically there are a small number of feed failures. That is, there are instances where the spring platen has failed to advance a film unit to the forwardmost exposure position indicating that the leading and trailing edges of a film unit may be in binding relation with the interior surfaces of the leading and trailing end walls of the container.
Therefore, it is an object of the present invention to improve the spread control structure thereon while at the same time maintaining certain common dimensional characteristics of the film container so that it is compatible with existing cameras.
Also, it is another object of the invention to improve the structure of the film container to facilitate the upward feeding of a stack of film units to the forwardmost position by a spring platen located in the film container.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.